1. Field of the Invention
The present invention relates to permeable membrane systems for the production of high purity gases.
2. Description of the Background
Membrane processes are presently employed for a wide variety of gas separations. Generally, in these processes, a feed stream is brought into contact with the surface of the membrane, wherein the more readily permeable component, such as oxygen in the case of air separation, is recovered at low pressure whereas the less readily permeable component, such as nitrogen in the case of air feed, is collected as a non-permeate stream at a pressure close to feed pressure.
The membrane systems manufactured today are economically viable when providing a non-permeate stream which is enriched in one of the feed components in the low purity range. However, gases of higher purity are required on the market.
Added to that, in many cases, membranes exhibit small defects, such as microleaks or flow deviation from ideality because of manufacturing difficulties. Consequently, module performances are not as good as would be expected in the absence of such defects. Moreover, the deviation from ideal performance becomes greater when the produced gas purity increases, which makes high purity gas production at low cost difficult.
Conventional systems for the production of high purity gas, which satisfy market demand are either Pressure Swing Adsorption devices (PSA) or cryogenic devices or hybrid systems using both membranes and PSA, or membranes or PSA associated with a reactor, for example, where the oxygen residue in the nitrogen permeate (or non-permeate) is reacted with hydrogen to form water which must then be eliminated. However, such hybrid systems are complicated and costly and do not offer the simplicity of membrane systems.
In an effort to improve membrane systems, the use of two membrane units has been proposed.
For example, a simple cascade process has been proposed where the non-permeate stream of a first stage is employed as a feed stream of a second stage permeator. Such a system is illustrated in FIG. 1, and is disclosed in U.S. Pat. No. 4,894,068.
Also, a recycle cascade process has been proposed where the permeate flow of a second stage is mixed with a feed stream of a first stage before a compression step. Such a system is illustrated in FIG. 2, and is disclosed in U.S. Pat. Nos. 4,180,388; 4,180,552; and 4,119,417.
On the other hand, other multistage processes have been described in the literature for the recovery of a permeate stream, such as continuous column processes, stripper processes and parallel processes.
The simple cascade process of FIG. 1 does not take advantage of the permeate stream of the second stage when this gas is enriched in less readily permeating gas when compared to the feed air or with the first stage permeate stream. As applied to the production of high purity nitrogen, as in U.S. Pat. No. 4,894,068, the simple cascade process is economically limited to only the small capacities of laboratory scale due to high energy consumption.
The recycle cascade process uses the permeate stream by mixing it with the feed stream before compression at high energy cost, and increased complexity.
Hence, a need continues to exist for a membrane process for the production of a high purity non-permeate stream which reduces the required investment and expenditure of energy. In particular, a need continues to exist for such a process for the production of high purity nitrogen on a large scale.